The term "communication system" covers a variety of networks, devices, and systems that provide communication between a plurality of nodes. One such communication system is a two-way wireless communication system as shown in FIG. 1. The two-way wireless communication system 10 includes a system controller 12, a switch 14, a plurality of base stations 16-20, a plurality of communication resources 26-26, and a plurality of communication units 28-32. The plurality of base stations 16-20 are coupled to the switch 14 via wireline links 34-38. These wireline links 34-38 are typically T1 links which have a bandwidth of approximately 1.5 megahertz and a capacity of 1.5 Mbps (mega-bits per second).
In operation, a communication unit transmits an inbound signaling word (ISW) via a wireless communication resource 26-26, which has been designated as a control channel. One of the base stations 16-20 receives the ISW and transmits it via the T1 link 34-38 to the switch 14. The switch provides the information to the system controller 12 which determines the validity of the request. For a valid request, the system controller grants the requested service, which may be a two-way wireless communication, a group call, a private call, or a telephone interface. Note that the communication units 28-32 may be any type of two-way wireless communication device, such as a mobile radio, a portable radio, a cellular telephone, or a cellular/mobile/portable radio telephone.
Having granted the request, the system controller 12 allocates a wireless communication resource 26-26 to the requesting communication unit. This information is sent out via the switch 14, one of the T1 links 34-38, and a base station 16-20. With the allocation of a wireless communication resource, the requesting communication unit may perform its wireless, or RF (Radio Frequency), communication.
While the two-way wireless communication system 10 of FIG. 1 provides a variety of services to a communication unit, or subscriber unit, the infrastructure requires high bandwidth wireline transmission links between the switch 14 and the base stations 16-20. As shown, these high bandwidth transmission links are typically T1 links which provides transmission capabilites of 1.5 Mbps. Alternatively, the links could be microwave links that provide an equivalent bandwidth as the T1 links.
Regardless of the type of link used, it must have a large bandwidth to convey large amounts of information between a base station and the switch. Because of the large amounts of information being conveyed, low frequency transmission lines, such as twisted pair copper telephone lines, cannot be used for these interconnections. Typically, a twisted pair copper wire telephone line has a bandwidth of 4 kilohertz. In two-way wireless communication systems, the intercoupling between the switch and base stations requires at least a 20 kilohertz bandwidth for a single data transfer. If multiple data transfers are occurring simultaneously, which is the typical case, the bandwidth requirements increase accordingly. Thus, based on these bandwidth requirements, the twisted pair copper telephone line does not provide adequate bandwidth.
Another type of communication system is illustrated in FIG. 2 as a one-way wireless communication system 40. The one-way wireless system 40, which may be a paging system, includes a system controller 42, a switch 44, wireline links 46-50, a plurality of transmitters 52-56, a plurality of communication resources 58-62, and a plurality of communication units 64-68 or pagers. In operation, the system controller receives a request to transmit a page to at least one of the plurality of communication units 64-68. Upon receiving this request, the system controller 42 generates a paging message which is transmitted to the switch 44 and subsequently routed via one of the wireline links 46-50 to the appropriate transmitter. Upon receiving the paging message, the transmitter transmits the message to the paging unit via one of the RF communication resources.
As with the two wire two-way wireless communication system 10 of FIG. 1, the one way wireless system 40 requires a high bandwidth wireline link between the switch and the transmitters. Thus, a twisted pair copper telephone line, which has a bandwidth of approximately 4 kilohertz, will not provide the needed transmission capacity of the one-way wireless communication system 40.
The two-way wireless system 10 and the one-way wireless system 40 each provide the communication unit with unique features. These unique features center around the fact that these wireless systems can transmit one communication to a plurality of receiving units. In other words, such wireless systems support one-to-many communications and/or many-to-one communications.
To enhance the transmit capabilities of a twisted pair copper telephone line, several techniques have been developed. For example Integrated Services Digital Network (ISDN) extends the bandwidth of a twisted pair copper telephone line from 4 kilohertz up to 200 kilohertz, which provides a transmission capability of 160 Kbps. Motorola part numbers MC145474, and MC145472 provide ISDN services. While these devices increase the bandwidth of a twisted pair telephone line, they are intended for one-to-one telephone communications. For example, the ISDN chips may be used for video telephone conferencing, facsimile transmissions, and pair gain transmissions, where pair gain transmission is a multiplexing technique of several telephone calls on a single line.
Another technique which increases the transmission capabilities of a twisted pair copper telephone line is Asymmetrical Digital Subscriber Loop, or Link, (ADSL). An ADSL device increases the bandwidth of the twisted pair telephone line up to 1.1 megahertz, which provides transmission capabilites up to 9 Mbps. Similar to the ISDN technique, the ADSL technique is used for one-to-one communications and provides additional bandwidth over the ISDN devices.
FIG. 3 illustrates a typical ADSL two wire system. The ADSL two wire system 70 includes a video server 72, which may be a camera, an asynchronous transfer mode (ATM) switch 74, an ADSL transceiver 76, a splitter 78, a plain old telephone service (POTS) switch 80, a twisted pair copper wire telephone line 82, a second splitter 84, a telephone 86, a second ADSL transceiver 88, and a television monitor 90. In general, the first and second ADSL transceivers 76 and 88 communicate to establish a spectral response of the telephone line 82. Having exchanged this information, a transmission can begin. In operation the video camera 72 will receive an image for teleconferencing and convert that image into digital information. The camera 72 routes the digital information to the ATM switch 74, which, in turn, mutes the digital information to the ADSL transceiver 76. The ADSL transceiver converts the digital information into a Discrete Multi-Tone (DMT) symbol and conveys the DMT symbol to the other ADSL transceiver via the splitters 78, 84 and the telephone line 82. Upon receiving the DMT symbol, the second ADSL transceiver 88 recaptures the digital information and routes the digital information to the TV monitor 90.
In addition to transmitting high bandwidth digital information, the ADSL two wire system 70 can also support regular telephone communications or POTS. This is done via the splitters 78 and 84 which route low frequency or POTS signals to the telephone 86 or the POTS switch 80 while routing the higher frequency signals to the ADSL transceivers.
FIG. 4 illustrates a frequency spectrum of carrier channels used in an ADSL system. The low frequency range, 0-4 kilohertz, is reserved for plain old telephone system (POTS) transmissions. The high frequency range, from 25 kilohertz to 1.1 megahertz, is used for ADSL transmissions. With this defined separation, the splitters 78, 84 of FIG. 3 can easily separate the low frequency signals from the high frequency signals.
The high frequency range of FIG. 4 is divided into 256 carrier channels separated by approximately 4 kilohertz. The first 32 carrier channels in the range from 25 kilohertz to 138 kilohertz are full duplex channels while the 224 channels in the frequency range from 138 kilohertz to 1.1 megahertz are half duplex channels. For the 32 carrier channels in the full duplex range, echo cancellation must be incorporated in the splitters to ensure proper reception and transmission of the signals.
Each carrier channel can support up to 15 bits of QAM information. The actual amount of bits a carrier channel supports varies, however, due to the spectral response of the telephone line. For example, one carrier channel may be able to accommodate 15 bits while another may be only able to accommodate 4 bits. Due to ADSL requirements the minimum amount of bits a carrier channel can support, referred to as bit loading, is 2 bits and still carry data.
To determine the spectral response of the telephone line, the ADSL transceivers use an algorithm as shown in FIG. 5. As shown, a first ADSL transceiver will transmit a wide band test signal to a second ADSL transceiver. Upon receipt, the second ADSL transceiver evaluates the received signal to determine the spectral response of the telephone line. Having the spectral response, the second ADSL generates a bit loading table and sends the bit loading table to the first ADSL transceiver. The bit loading table includes, for each carrier channel, a number of bits that the carrier channel can support.
FIG. 6 illustrates the transmit portion of the ADSL transceiver. As shown, the ADSL transmitter includes a multiplexer which receives a plurality of inputs via T1 links, ADSL control, an ISDN connection, and an HO link. Based on the ADSL control, the multiplexer provides one of the inputs to a constellation encoder via a fast path or an interleave path. The fast path includes a scramble cyclic redundancy check (CRC) block which is coupled to a forward error correction (FEC) block. The interleave path includes a scramble CRC, a forward error correction block, and an interleave block. The path selected depends on the level of burst error correction needed. If less error correction is needed, the fast path is selected, otherwise the interleave path is selected.
The constellation encoder encodes the received signals based on the bit loading table and an encoding sequence to produce an encoded data stream. The encoded data stream is then provided to the Discrete Multi-Tone (DMT) modulator which produces a DMT symbol from the encoded data stream. The DMT symbol is then transmitted to the receiver of the other ADSL transceiver via the telephone line.
FIG. 7 illustrates the receiver portion of the ADSL transceiver. As shown, the ADSL receiver includes a DMT demodulator which demodulates the DMT symbol to produce a demodulated signal. The demodulated signal is then provided to the constellation decoder which decodes the signal based on the bit loading information and a decoding sequence to recapture the transmitted data stream. The recaptured data stream is then provided to the demultiplexer via a fast path or deinterleave path. The de-multiplexer then provides recaptured data to the appropriate output line.
While the ADSL system increases the bandwidth of a telephone line, up to 1.1 megahertz, it is designed for one-to-one communications and not one-to-many or many-to-one communications. Therefore, a need exists for a one-to-many and/or many-to-one communication system infrastructure that utilizes existing telephone lines while providing the highly reliable service subscribers of wireless communication systems expect.